Self-driving circuit for a DC/DC converter

ABSTRACT

The present invention provides a self-driving circuit for DC/DC converter of a low voltage, high current, and high power density. The converter comprises a transformer, output rectification portion SR 1  and voltage clamping. The first configuration of the self-driving circuit consists of resisters R a1 , R a2 , capacitors C a1 , C a2 , transistors Q a1 , Q a2 ; and the second configuration consists of Da, small power MOS transistor SRa, an auxiliary winding Nsa, a delay driving circuit and a isolating differential circuit. The self-driving circuit of the present invention may reduce the cross-conductive loss, and increase the converting efficiency.

The present invention relates to a self-driving circuit for a DC/DCconverter of low voltage, high current, and high power density.

With the rapid development of high technologies, such as,communications, remote sensing, electronic computers, and electronicinstrument, the requirement of power supplies of such electronicequipment has increased accordingly. DC/DC converter of low voltage,high current, and high power density is the core technology of the powersupplies for supper integrated circuits and high-speed centralprocessors. To meet high efficiency and high power density requirement,the auxiliary (secondary) side of such DC/DC converters shall usesynchronous rectifying MOSFET transistor in place of Schottky diode forrectification in order to reduce power loss. However, for a synchronousrectifying MOSFET, the gate thereof needs a corresponding drive circuitto stimulate. In order to prevent cross-conductive losses, therequirement of time sequence of the drive circuit is very high. Theexisting drive circuits utilize external driving technology, but itscontrol is too complicated and the cost is high.

For a converter having only one synchronous rectifying MOSFET at thesecondary side, such as a backward stimulating circuit, the synchronousrectifying MOSFET cannot be driven directly by the waveform of thesecondary side of the transformer. Otherwise, the transistor would bedamaged for failure to shut down. For instance, FIG. 1a shows atraditional backward stimulating circuit. The voltage waveform of itssecondary side is shown in FIG. 1b. If such a waveform of the secondaryside is used to drive the gate of SR₁, as shown in FIG. 2, SR₁ mayhardly be turned off owing to V_(gs1)=V_(o), and there will beshort-circuit in the secondary side to cause losses when S turns on.

If an external auxiliary winding is used for self-driving, theconverting efficiency may reduce dramatically because it is hard tocontrol the cross-conducting time. The efficiency is normally lower thanthat of using Schottky diode. For instance, FIG. 3 shows a self-drivingcircuit using an external auxiliary winding Nsa to drive SR₁. When Sturns off and the secondary side voltage changes to positive at top andnegative at bottom, SR₁ turns on, and the energy of the secondary sideof the transformer will be provided to the load through SR₁; when Sturns on, S and SR₁ may be on at the same time because it will needsometime for the secondary side voltage to change to negative at bottomand positive at top such that the secondary side would beshort-circuited. Although this time is relatively short, itscross-conductive loss is rather high. If serious, it may damage S andSR₁. Even under the normal operation, the converting efficiency canhardly be increased. Therefore, to improve the converting efficiency,the self-driving circuit should be modified.

Therefore, the object of the present invention is to solve the existingproblem in the self-driving circuit of the main circuit of a DC/DCconverter of low voltage and high current, and to provide a self-drivingcircuit for the converter that has reduced cross-conductive loss, simplestructure, and low cost.

The present invention is realized through the following technicalembodiments. In the first configuration of the self-driving circuit ofthe DC/DC converter of the present invention, the rectification portionof the converter comprises a synchronous rectifying MOS transistor(SR₁), wherein the self-driving circuit is composed of two resisters(R_(a1), R_(a2)), two capacitors (C_(a1), C_(a2)), a PNP transistor(Q_(a1)) and a NPN transistor (Q_(a2)). The resister (R_(a1)) and thecapacitor (C_(a1)) are connected in parallel, and an end of the parallelconnection of the register (R_(a2)) and capacitor (C_(a2)) connectedwith the base of the transistor (Qa) and the other end connected with anend of parallel connected resister (R_(a2)) and capacitor (C_(a2)), andwith the positive end of the transformer winding (Ns) and the drain endof the rectifying MOS transistor (SR₁). The other end of the parallelconnection of the resister (R_(a2)) and capacitor (C_(a2)) is connectedwith the base end of the transistor (Q_(a2)). The emitter of thetransistor (Q_(a2)) is connected with the source end of the MOStransistor (SR₁), while its collector is connected the collector of thetransistor (Q_(a1)) and the gate of the MOS transistor (SR₁). Theemitter of the transistor (Q_(a1)) is connected with the negative end ofthe winding (Ns).

In the second configuration of the self-driving circuit of the DC/DCconverter of the present invention, the rectification portion of theconverter comprises a synchronous rectifying MOS transistor (SR₁),wherein the self-driving circuit is composed of a diode (Da), a smallpower MOS transistor (SRa), an auxiliary winding (Nsa), a time delaydriving circuit, and an isolating differential circuit. The delaydriving circuit and the isolating differential circuit are connectedwith each other. An end of the isolating differential circuit isconnected with the gate of the small power MOS transistor SRa. Thepositive end of the auxiliary winding (Nsa) is connected with the sourceend of the small power MOS transistor (SRa) and the source end of thesynchronous rectifying MOS transistor (SR₁), while its negative end isconnected with the anode of the diode Da. The cathode of the diode Da isconnected with the gate of the synchronous rectifying MOS transistor(SR₁) and the drain end of the small power transistor (SRa).

The isolating differential circuit may be composed of the windings(N_(pa1)) and (N_(pa2)) of the transformer, two capacitors, tworesisters, and a diode. The winding N_(sa1) is connected, through thecapacitor, with the parallel-connected resister and diode.

The time delay driving circuit is composed of a delay circuit and adriving circuit, wherein an example of the delay circuit is formed byconnecting the diode and resister in parallel, and then connected inserial to a ground capacitor.

The DC/DC converter is a double backward converter including windings(Np, Ns) and power MOS transistors (S₁, S₂). The positive end of thewinding (Np) is connected with the source end of the power MOStransistor (S₁), and the negative end of the winding (Np) is connectedto the drain end of the power MOS transistor (S₂). The delay drivingcircuit is connected with the gates of the power MOS transistors (S₁)and (S₂), respectively.

The DC/DC converter is a clamping backward converter of three windings(Nc, Np, Ns), including the windings (Nc, Np, Ns), power MOS transistor(S) and diode (Dc). The negative end of the winding (Np) is connectedwith the drain end of the power MOS transistor (S), and the positive endof the winding (Nc) is connected with the cathode of the diode (Dc). Thedelay driving circuit is connected with the gate of the power MOStransistor (S).

The DC/DC converter is a R.C.D. clamping backward converter includingwindings (Np, Ns), a power MOS transistor (S), a resister (Rc), a diode(Dc) and a capacitor (Cc). The negative end of the winding (Np) isconnected with the drain end of the power MOS transistor (S). The delaydriving circuit is connected with the gate of the power MOS transistor(S), and the negative end of the winding Np is connected with the anodeof the diode (Dc). An end of the parallel-connected capacitor (Cc) andresister (Rc) is connected with the cathode of the diode (Dc), while theother end is connected with the positive end of the winding (Np).

The DC/DC converter is an active clamping backward converter includingwindings (Np, Ns), power MOS transistors (S, Sc) and a capacitor (Cc).The positive end of the winding (Np) is connected through the capacitor(Cc) with the drain end of the power MOS transistor (Sc). The source endof the power MOS transistor (Sc) is connected with the drain end of thepower MOS transistor (S) and the negative end of the winding (Np). Thedelay driving circuit is connected with the gate of the power MOStransistor (S).

The converter is a diode clamping double backward converter includingwindings (Np, Ns), power MOS transistors (S₁, S₂) and diodes (D₁, D₂).The positive end of the winding (Np) is connected with the source end ofthe power MOS transistor (S₁), and the negative end of the winding (Np)is connected with the drain end of the power MOS transistor (S₂). Theanode of the diode (D₁) is connected with the negative end of thewinding (Np), and the cathode is connected with the drain end of thepower MOS transistor (S₁). The anode of the diode (D₂) is connected withthe source end of the power MOS transistor (S₂), and the cathode isconnected with the positive end of the winding (Np). The delay drivingcircuit is connected, respectively, with the gates of the power MOStransistors (S₁), (S₂).

The converter is an active clamping double backward converter includingwindings (Np, Ns), power MOS transistors (S₁, S₂), a capacitor (Cc) anda power MOS transistor (Sc). The positive end of the winding (Np) isconnected with the source of the power MOS transistor S₁), and thenegative end of the winding (Np) is connected with the drain end of thepower MOS transistor (S₂). The capacitor (Cc) and the power MOStransistor (Sc) are connected in serial, and then connected parallel thewinding (Np), with its two ends connected with positive and negativeends of the winding (Np), respectively. The delay driving circuit isconnected with the gates of the power MOS transistors (S₁) and (S₂),respectively.

The present invention utilizes certain small power resistance andcapacitance elements, diodes, transistors or field effect transistors torealize the equivalent self-driving technology. Thus, it ensures thereliable turn-on and turn-off of SR1, and at the same time it ensuresthe minimum cross-conductive loss and high converting efficiency.

The present invention will be further explained through the followingembodiments in conjunction with the accompanying drawings.

FIG. 1a is a normal backward converter circuit;

FIG. 1b is the waveform of the secondary side voltage of the abovecircuit;

FIG. 2 is a direct self-driving circuit;

FIG. 3 is a known self-driving circuit;

FIG. 4a is the first self-driving circuit of the present invention;

FIG. 4b is the second self-driving circuit of the present invention;

FIG. 5a is an embodiment of a double backward converter using the firstself-driving circuit;

FIG. 5b is an embodiment of a double backward converter using the secondself-driving circuit;

FIG. 6a is an embodiment of a three-winding clamping backward converterusing the first self-driving circuit;

FIG. 6b is an embodiment of a three-winding clamping backward converterusing the second self-driving circuit;

FIG. 7a is an embodiment of a R. C. D. clamping backward converter usingthe first self-driving circuit;

FIG. 7b is an embodiment of a R. C. D. clamping backward converter usingthe second self-driving circuit;

FIG. 8a is an embodiment of an active clamping backward converter usingthe first self-driving circuit;

FIG. 8b is an embodiment of an active clamping backward converter usingthe second self-driving circuit;

FIG. 9a is an embodiment of a diode clamping double backward converterusing the first self-driving circuit;

FIG. 9b is an embodiment of a diode clamping double backward converterusing the second self-driving circuit;

FIG. 10a is an embodiment of an active clamping double backwardconverter using the first self-driving circuit;

FIG. 10b is an embodiment of an active clamping double backwardconverter using the second self-driving circuit;

FIG. 11 shows a typical waveform of the converter using the drivingcircuit of FIG. 4 (a); and

FIG. 12 shows a typical waveform of the converter using the drivingcircuit of FIG. 4 (b ).

The operation principle of the two self-driving circuits of the presentinvention is explained hereinafter. FIG. 4 (a) is a non-isolatingself-driving circuit of a normal backward converter. Windings Np and Nsare connected in a manner of non-identical names, the rectificationportion of the converter comprises a synchronous rectifying MOStransistor SR₁. The self-driving circuit is composed of two resistersR_(a1), R_(a2), two capacitors C_(a1), C_(a2), a PNP transistor Q_(a1)and a NPN transistor Q_(a2). The configuration is as follows: theresister R_(a1) and the capacitor C_(a1) are connected in parallel, andan end of the parallel connection is connected with the base of thetransistor Q_(a1), and the other end thereof is connected with an end ofthe parallel connection of the register R_(a2) and capacitor C_(a2), andwith the positive end of the transformer winding Ns and the drain end ofthe rectifying MOS transistor SR₁. The other end of the parallelconnection of the resister R_(a2) and capacitor C_(a2) is connected withthe base end of the transistor Q_(a2). The emitter of the transistorQ_(a2) is connected with the source end of the MOS transistor SR₁, whileits collector is connected the collector of the transistor Q_(a1) andthe gate end of the MOS transistor SRI. The emitter of the transistorQ_(a1) is connected with the negative end of the winding Ns.

Through R_(a1), C_(a1), and Q_(a1), SR₁ may turn on after S turns off.Through R_(a2), C_(a2), and Q_(a2), SR₁ remains off when S is on.

When the self-driving circuit of FIG. 4a is used, the typical operationwaveform of various voltages and current of the converter is shown inFIG. 11.

At t=t₁, the main switch S turns off; at this time, the waveform of thesecondary side voltage changes from positive at bottom and negative attop to positive at top and negative at bottom. Q_(a1) turns on quicklythrough R_(a1). and C_(a1) V_(aSR1) changes to V_(O) so as to turn onSR₁, and the energy of the secondary side of the transformer is providedto the load through SR₁. At t=t₂, the main switch S turns on again. Thewaveform of the secondary side changes from to positive and bottomnegative to top negative and bottom positive, and Q_(a2) turns onquickly through R_(a2), C_(a2). V_(gSR1) changes to zero so as to turnoff SR₁, and the primary side of the transformer stores energy, and thecapacitor C provides power to the load. The accelerate circuit formed ofR_(a2), C_(a2) enables that Q_(a2) turns on quicker than SR₁, so as toensure the quick discharge of the gate voltage of SR₁. By simplycarefully selecting R_(a2), C_(a2), it may dramatically reduce thecross-conductive loss between SR₁ and S to increase the convertingefficiency. R_(a2) and C_(a2) can hardly be optimized when the frequencyis relatively high because there is a storing time when Q_(a2) turnsoff.

Accordingly, the switch frequency of such technology should be lowerthan 250kHz. When the switch frequency of the converter is relativelylow, such technology may achieve very high converting efficiency.

FIG. 4b is an isolating self-driving circuit of the normal backwardconverter. The circuit of FIG. 4b is to utilize an auxiliary windingNsa, a diode Da, a small power MOSFET (SRa), a set of isolating pulsecontrol circuit and a time delay driving circuit to accomplish thefunction of R_(a1), C_(a1), Q_(a1) and R_(a2), C_(a2), Q_(a2) of FIG.4a. The rectification portion of the converter comprises a synchronousrectifying MOS transistor SR₁, and the self-driving circuit is composedof a diode Da, a small power MOS transistor SRa, an auxiliary windingNsa, a delay driving circuit and an isolating differential circuit. Thestructure is as follows: the delay driving circuit and the isolatingdifferential circuit are connected; an end of the isolating differentialcircuit is connected with the gate of the small power transistor SRa;the positive end of the auxiliary winding is connected with the sourceof the small power MOS transistor SRa, and the negative end is connectedto the anode of the diode Da. The cathode of the diode Da is connectedwith the gate of the synchronous rectifying MOS transistor SR₁ and thedrain end of the small power transistor SRa.

The function of the delay driving circuit is to let SR1 turn off alittle bit earlier than the turn-on of S so as to control the best delaytime interval and to increase the efficiency of the converter. Thetypical waveform thereof is shown in FIG. 12.

The turning-on of SR1 is realized by the external auxiliary winding Nsaand the diode Da, and the turning-off thereof is realized by a smallpower MOSFET Sa, a delay driving circuit and an isolating differentialcircuit. Through the delay driving circuit, SR1 is adjustably turned offbefore S turns on so as to minimize the cross-conductive loss and tomaximize the efficiency of the converter. This circuit differs from FIG.4a as follows: (1) addition of certain small auxiliary circuit (such asDa, Sa, a delay circuit and an isolating differential circuit); (2)adjustable dead region of the cross-conduction; (3) flexibility ofoptimization of power stage.

The advantage of this kind of self-driving circuit is more common, andhas no restriction to the switch frequency of the converter. Thedisadvantage is the little complicated structure, and requiresisolation. However, the size of the all of components is still smallbecause only pulse is required to drive Sa.

The present invention uses some additional small power elements, suchthat the DC/DC converter of only one synchronous rectifying MOSFET(e.g., backward converter) may also use self-driving synchronousrectification technology.

The two inventive technology of FIG. 4 can be broadly adopted. This canbe seen in FIG. 5 to FIG. 10 for other embodiments. The advantages ofthem are described as follows:

FIG. 5 is the application of the two circuits of the present inventionin a double backward stimulating circuit. The DC/DC converter includeswindings Np, Ns and power MOS transistors S₁, S₂. The positive end ofthe winding Np is connected with the source end of the power MOStransistor S₁, and the negative end of the winding Np is connected withthe drain of the power MOS transistor S₂. The delay driving circuit isconnected, respectively, to the gate of the power MOS transistors S₁,S_(2.)

FIG. 6 to FIG. 8 are the applications of these two circuits of thepresent invention in backward converters of various clamping circuits.FIG. 6 is such an application in a three-winding clamping backwardstimulating circuit. This three-winding (Nc, Np, Ns) clamping backwardconverter includes windings Np, Ns, Nc, a power MOS transistor S and adiode Dc. The negative end of the winding Np is connected with the drainof the power MOS transistor S, and the positive end of the winding Nc isconnected with the cathode of the diode Dc. The delay driving circuit isconnected with the gate of the power MOS transistors.

FIG. 7 is such an application in a R.C.D. clamping backward stimulatingcircuit. The R.C.D. clamping backward converter includes windings Np,Ns, a power MOS transistor S, a resister Rc, a diode Dc and a capacitorCc. The negative end of the winding Np is connected with the drain endof the power MOS transistor S. The delay driving circuit is connectedwith the gate end of the power MOS transistor S, and the negative end ofthe winding Np is connected with the anode of the diode Dc. An end ofparallel-connected capacitor Cc and resister Rc is connected with thecathode of the diode Dc, and the other end is connected with thepositive end of the winding Np.

FIG. 8 is such an application in an active clamping backward stimulatingcircuit. The active clamping backward converter includes windings Np,Ns, power MOS transistors S, Sc and a capacitor Cc. The positive end ofthe winding Np is connected through the capacitor Cc with the drain endof the power MOS transistor Sc. The source end of the power MOStransistor Sc is connected with the drain end of the power MOStransistor S and the negative end of the winding Np. The delay drivingcircuit is connected with the gate of the power MOS transistor S.

FIG. 9-FIG. 10 are the applications of these two self-driving circuitsof the present invention in double backward stimulating circuit variousclaiming circuits. FIG. 9 is such an application in a diode clampingdouble backward circuit. The diode clamping double backward converterincludes windings Np, Ns, power MOS transistors S₁, S₂ and diodes D₁,D₂. The positive end of the winding Np is connected with the source endof the power MOS transistor S₁, and the negative end of the winding Npis connected with the drain end of the power MOS transistor S₂. Theanode of the diode D₁ is connected with the negative end of the windingNp, and the cathode is connected with the drain end of the power MOStransistor S₁. The anode of the diode (D₂) is connected with the sourceend of the power MOS transistor (S₂), and the cathode is connected withthe positive end of the winding (Np). The delay driving circuit isconnected, respectively, with the gates of the power MOS transistors S₁,S₂.

FIG. 10 is such an application in an active clamping double backwardstimulating circuit. The active clamping double backward converterincludes windings Np, Ns, power MOS transistors S₁, S₂, a capacitor Ccand a power MOS transistor Sc. The positive end of the winding Np isconnected with the source end of the power MOS transistor S₁, and thenegative end of the winding Np is connected with the drain end of thepower MOS transistor S₂. The capacitor Cc and the power MOS transistorSc are connected in serial, and then connected parallel with the windingNp, with its two ends connected with positive and negative ends of thewinding Np, respectively. The delay driving circuit is connected withthe gates of the power MOS transistors S₁, S₂, respectively.

The two self-driving circuit of the present invention have been provedthrough experiments. The first embodiment has been used in a DC/DC powersupply (using a three-winding clamping backward circuit) of 40-60Vdirect current input, and 1.2-1.65V@35A direct current output. Theefficiency of the power stage reaches 84%. The second embodiment hasbeen used in a DC/DC power supply (using a three-winding clampingbackward circuit) of 36-72V direct current input and 5V@30A directcurrent output. The efficiency of the power stage reaches 90%.

The present invention has been explained through the embodiments.However, the present invention is not limited thereto. Any improvementand substitution should be viewed within the scope of the protection ofthe present invention provided that they are not apart from the spiritsand contents of the present invention.

What is claimed is:
 1. A self-driving circuit of a DC/DC converterhaving a transformer and a rectification portion, said rectificationportion including at least a synchronous rectifying MOS transistor,characterized in that the self-driving circuit comprises a first and asecond resisters, a first and a second capacitors, a PNP transistor anda NPN transistor; the first resister and the first capacitor beingconnected in parallel, and an end of the parallel connection beingconnected with the base of said PNP transistor, and the other end of theparallel connection connected with an end of parallel connected thesecond resister and the second capacitor, and connected with thepositive end of the transformer secondary winding and the drain end ofthe synchronous rectifying transistor; the other end of said parallelconnection of the second resister and second capacitor being connectedwith the base end of said NPN transistor; the emitter of said NPNtransistor being connected with the source end of the synchronousrectifying transistor, while the collector of said NPN transistor isconnected with the collector of said PNP transistor; the emitter of saidPNP transistor being connected with the negative end of said transformersecondary winding.
 2. The self-driving circuit of claim 1, characterizedin that said DC/C converter is a double backward converter, the positiveend of the primary winding of said transformer being connected with thesource end of a first power MOS transistor, and the negative end of theprimary winding of said transformer being connected to the drain end ofa second power MOS transistor.
 3. The self-driving circuit of claim 1,characterized in that said DC/DC converter is a three-winding clampingbackward converter, including a first, a second, and a third windings, apower MOS transistor and a diode, the negative end of the first windingbeing connected with the drain end of the power MOS transistor, and thepositive end of the second winding connected with the cathode of saiddiode, and the third winding being the secondary winding of saidtransformer.
 4. The self-driving circuit of claim 1, characterized inthat said DC/DC converter is a R.C.D. clamping backward converter,including a first winding and a second winding, a power MOS transistor,a resister, a diode and a capacitor, the negative end of the firstwinding being connected with the drain end of the power MOS transistor,and with the anode of said diode, an end of parallel-connected saidcapacitor and said resister being connected with the cathode of saiddiode, while the other end of the parallel connection is connected withthe positive end of the first winding, said second winding being thesecondary winding of said transformer.
 5. The self-driving circuit ofclaim 1, characterized in that said DC/DC converter is an activeclamping backward converter, including a first and a second windings, afirst and a second power MOS transistors and a capacitor, the positiveend of said first winding being connected through said capacitor withthe drain end of the second power MOS transistor, the source end of thesecond power MOS transistor being connected with the drain end of thefirst power MOS transistor and the negative end of the first winding,and said second winding being the secondary winding of said transformer.6. The self-driving circuit of claim 1, characterized in that saidconverter is a diode clamping double backward converter, including afirst and a second windings, a first and a second power MOS transistorsand a first and a second diodes, the positive end of the first windingbeing connected with the source end of the first power MOS transistor,and the negative end of the first winding connected with the drain endof the second power MOS transistor, the anode of the first diode beingconnected with the negative end of the first winding, and its cathodeconnected with the drain of the first power MOS transistor, the anode ofthe second diode being connected with the source end of the second powerMOS transistor, and its cathode connected with the positive end of thefirst winding, and said second winding being the secondary winding ofsaid transformer.
 7. The self-driving circuit of claim 1, characterizedin that said converter is an active clamping double backward converter,including a first and a second windings, a first and a second power MOStransistors, a capacitor and a third power MOS transistor, the positiveend of the first winding being connected with the source of the firstpower MOS transistor, and the negative end of the first windingconnected with the drain end of the second power MOS transistor, thecapacitor and the third power MOS transistor being connected in serial,and then connected parallel with the first winding, with its two endsconnected with positive and negative ends of the first winding,respectively, and said second winding being the secondary winding ofsaid transformer.
 8. The self-driving circuit of claim 2, characterizedin that said DC/DC converter is a double backward converter, thepositive end of the primary winding of said transformer being connectedwith the source end of a first power MOS transistor, and the negativeend of the primary winding of said transformer being connected to thedrain end of a second power MOS transistor.
 9. A self-driving circuitfor a DC/DC converter having a transformer and a rectification portion,said rectification portion including a synchronous rectifying MOStransistor, characterized in that the self-driving circuit comprises adiode, a small power MOS transistor, an auxiliary winding, a time delaydriving circuit, and an isolating differential circuit, said time delaydriving circuit and said isolating differential circuit being connectedwith each other; the other end of said isolating differential circuitbeing connected with the gate of the small power MOS transistor; thepositive end of the auxiliary winding being connected with the sourceend of the small power MOS transistor, while its negative end isconnected with the anode of the diode, the cathode of said diode beingconnected with the gate of the synchronous rectifying MOS transistor andthe drain end of the small power transistor.
 10. The self-drivingcircuit of claim 9, characterized in that said DC/DC converter is adouble backward converter, the positive end of the primary winding ofsaid transformer being connected with the source end of a first powerMOS transistor, and the negative end of the primary winding of saidtransformer being connected to the drain end of a second power MOStransistor, the gates of said first and second power MOS transistorsbeing connected to the time delay driving circuit respectively.
 11. Theself-driving circuit of claim 10, characterized in that said synchronousrectifying MOS transistor turns off before the first power MOStransistor, connected to the primary side of said transformer, turns on.12. The self-driving circuit of claim 9, characterized in that saidDC/DC converter is a three-winding clamping backward converter,including a first, a second, and a third windings, a power MOStransistor and a diode, the negative end of the first winding beingconnected with the drain end of the power MOS transistor, and thepositive end of the second winding connected with the cathode of saiddiode, the gate of said power MOS transistor connected with said timedelay driving circuit, and the third winding being the secondary windingof said transformer.
 13. The self-driving circuit of claim 9,characterized in that said DC/DC converter is a R.C.D. clamping backwardconverter, including a first winding and a second winding, a power MOStransistor, a resister, a diode and a capacitor, the negative end of thefirst winding being connected with the drain end of the power MOStransistor, and with the anode of said diode, the gate of said power MOStransistor connected with said time delay driving circuit, an end ofparallel-connected said capacitor and said resister being connected withthe cathode of said diode, and the other end of the parallel connectionbeing connected with the positive end of the first winding, said secondwinding being the secondary winding of said transformer.
 14. Theself-driving circuit of claim 9, characterized in that said DC/DCconverter is an active clamping backward converter, including a firstand a second windings, a first and a second power MOS transistors and acapacitor, the positive end of said first winding being connectedthrough said capacitor with the drain end of the second power MOStransistor, the source end of the second power MOS transistor beingconnected with the drain end of the first power MOS transistor and thenegative end of the first winding, said time delay driving circuitconnected with the gate of said power MOS transistor, and said secondwinding being the secondary winding of said transformer.
 15. Theself-driving circuit of claim 9, characterized in that said converter isa diode clamping double backward converter, including a first and asecond windings, a first and a second power MOS transistors and a firstand a second diodes, the positive end of the first winding beingconnected with the source end of the first power MOS transistor, and thenegative end of the first winding connected with the drain end of thesecond power MOS transistor, the anode of the first diode beingconnected with the negative end of the first winding, and its cathodeconnected with the drain of the first power MOS transistor, the anode ofthe second diode being connected with the source end of the second powerMOS transistor, and its cathode connected with the positive end of thefirst winding, said time delay driving circuit being connected,respectively, with the gates of said first and second power MOStransistor, and said second winding being the secondary winding of saidtransformer.
 16. The self-driving circuit of claim 9, characterized inthat said converter is an active clamping double backward converter,including a first and a second windings, a first and a second power MOStransistors, a capacitor and a third power MOS transistor, the positiveend of the first winding being connected with the source of the firstpower MOS transistor, and the negative end of the first windingconnected with the drain end of the second power MOS transistor, thecapacitor and the third power MOS transistor being connected in serial,and then connected parallel with the first winding, two ends of theparallel connection being connected with positive and negative ends ofthe first winding, respectively, said time delay driving circuit beingconnected, respectively, with the gates of said first and second powerMOS transistors, and said second winding being the secondary winding ofsaid transformer.
 17. The self-driving circuit of claim 9, characterizedin that the time delay driving circuit includes a delay circuit and adriving circuit, wherein the delay circuit is formed by connecting thediode and resister in parallel and then connected in serial to a groundcapacitor.